Math Is Fun Forum

"Casting out nines is an important tool in checking addition/multiplication."

ganesh, if it doesn't work all the time, and it seems to take as much time - or longer - than to just do the opposite, what makes it important? I think I understand HOW to do it, but I'm unsure why they should be learning this. 

At the fourth grade level, I doubt they would understand why this works (I don't even know why it works) sometimes.  And if it only works sometimes and not all the time - like doing the opposite operation, then of what use is it?